A wind turbine converts wind power into electrical energy through a generator driven by the blades of the wind turbine under windy conditions. In some wind turbines, the generator comprises a stator and a rotor both having multiphase windings, such as a double-fed induction generator, a cascade-regulated generator, an induction or a synchronic generator, or even a DC generator.
When having a rotor rotating in relation to a stator of a generator, electricity has to be transferred to or from the rotor. Electricity may be transferred by means of a slip ring assembly also called a rotary electrical interface, rotating electrical connector, collector, or swivel or electrical rotary joint, enabling the rotor to receive or return electricity while rotating in relation to the stator. When having a rotor with a multiphase winding, a control system within the wind turbine is able to regulate the frequency, so that the wind turbine produces at a predetermined frequency, e.g. approximately 50 Hz.
Typically, the slip ring assembly comprises a rotational slip ring unit rotating with the rotor and having conductive rings or bands. Furthermore, the slip ring assembly comprises brushes having sliding contact with the rings for transferring of electricity to the rotor. Thus, the electricity is transferred from the slip ring unit to the rotor windings in order to control the electrical field in the generator. The electricity is transferred to the slip ring unit by means of brushes brushing against the conductive rings, typically three conductive rings—one for each electrical phase. When the brushes brush against the rings, heat is generated due to the friction.
Tests have shown that the wear on the brushes can be substantially reduced if the surface of the brushes facing the rings is cooled down during the conduction of electricity. Replacing abraded brushes in a wind turbine is very costly and thus there is a need for increasing the service life of the brushes.
Many attempts to cool the brushes have been developed. One solution is shown in JP-53116406-A, in which air is led in through the centre of rotation and past the brushes before exiting the housing again. However, by blowing air into the housing of the slip ring assembly, the dust released from the brushes is spread to the generator where the dust has a harmful effect on the function of the slip ring.
In other solutions, the air is sucked out of the housing reducing the risk of the granite dust to be spread within the housing. However, the cooling is not very efficient and thus does not substantially increase the service life of the brushes.